1. Field of the Invention
The present invention relates to heart valve protheses, and more particularly, to suture rings for supporting heart valve protheses.
2. Description of the Prior Art
There are two types of heart valve prostheses, biological and mechanical. The medical indications for heart valve replacement are the same for both types. Examples include rheumatic heart disease, congenital anomalies, and myocardial infarction.
Unidirectional flow is the primary function of heart valve prostheses. This is usually accomplished by fashioning rigid or flexible leaflets, free to articulate within certain limitations, within an annular shaped frame, frequently referred to as an orifice ring. The restrained motion of these leaflets causes the flow to be essentially unidirectional, mimicking the natural function of native heart valves.
While the features of the present invention can be used in either biological or mechanical valves, for purposes of facilitating explanation thereof, the present prior art disclosure dissertation will be limited to mechanical valves, such as disclosed in U.S. Pat. Nos. 4,276,658-Hansen dated Jul. 7, 1981, 4,689,046-Bokros dated Aug. 25, 1987 and 4,950,287-Reif dated Nov. 12, 1991. The leaflets of mechanical valves are usually constructed of pyrolytic carbon or a composite of pyrolytic carbon and a substrate, such as graphite or titanium. The leaflets are typically constrained within an orifice ring also constructed of the same materials. In most cases the orifice ring is deformed in order to insert the leaflets during manufacture. Therefore, it is desirable for the orifice ring to be somewhat compliant. If the orifice ring is too stiff, a significant percentage may be permanently damaged during the insertion process. It is also desirable to maximize the internal diameter of the orifice ring, since this reduces the pressure gradient through the valve, reducing the work that the heart must perform during each stroke.
The orifice ring with the inserted leaflets is often referred to as a subassembly. The subassembly is usually attached to the heart by using a biocompatible fabric material, such as Dacron.TM.. The fabric material is usually purchased or fashioned into a tubular configuration. There are several methods of fixation of the fabric material to the subassembly. One possibility is disclosed in U.S. Pat. No. 3,781,969-Anderson dated Jan. 1, 1974, where the subassembly is placed inside of the fabric tube and a heat shrinkable plastic band is placed around the outside diameter of the fabric tube. The fabric material is then folded into an annular configuration often referred to as the suture ring. Sometimes annular shaped filler rings, often constructed of Teflon.TM. or Silastic.TM., are inserted within the folded portion of the fabric tube in order to make the suture ring larger and/or more compliant. It is desirable that a suture ring be rotatable relative to the subassembly, as this feature greatly facilitates implantation into the heart. The use of a heat shrinkable plastic band is one method of achieving rotatability.
Significant forces are applied to the suture ring during both the surgical implantation of the heart valve and during its service life in the body. These forces are transmitted to the leaflets via the orifice ring. It is possible, therefore, to damage the subassembly both during and after implantation. The use of a heat shrinkable plastic band requires that the orifice ring have substantial stiffness. This makes the insertion of the leaflets more difficult and reduces the internal diameter of the orifice ring, both of which are undesirable. Since pyrolitic carbon is a preferred material for the orifice ring, and since it is much more compliant than metal (about 7.5 times more compliant than steel and about 3.8 times more compliant than titanium), it is apparent that both of these problems can be overcome by using a metal stiffening ring around the outside diameter of the subassembly.
U.S. Pat. Nos. 5,071,431-Sauter et al dated Dec. 10, 1991, uses a continuous metal stiffening ring. The inside diameter of the stiffening ring is in direct proximity to the outside diameter of the subassembly, but not in direct contact with it. The inside diameter of the fabric tube is in direct contact with the outside diameter of the stiffening ring and continuous metal fastener bands are used at the proximal and distal ends of the stiffening ring in order to fix the fabric tube to the stiffening ring. This stiffening ring of U.S. Pat. No. 5,071,431 uses a metal split ring as means to prevent the stiffening ring from disengaging from the subassembly and to provide some control over the rotatability of the subassembly within the suture ring. In practice, the assignee of U.S. Pat. No. 5,071,431 with this stiffening ring uses a metal wire for this purpose. The outside diameter of the orifice has a small groove, the inside diameter of the locking ring has a similar small groove, and the metal wire passes within this potential resultant groove space. Therefore, the outside diameter of the orifice ring is constrained by the stiffening ring only over the small contact area from the wire. The disadvantage to this method of constraint is that it requires the orifice ring to be thicker than it would be if the constraint were to be applied over a larger portion of the external diameter of the orifice ring. This is because the leaflets transfer significant loads to the orifice ring, when the leaflets are in the closed position. For the same loading conditions from the leaflets, the larger the area of constraint on the outside diameter of the orifice ring, the lower the stress in the orifice ring. Increasing the thickness of the orifice ring results in a decrease in the inside diameter of the orifice ring, which is an undesirable effect.
Similar arguments can be used to discount the effectiveness of the continuous metal stiffening ring disclosed in U.S. Pat. No. 5,397,348-Campbell et al dated Mar. 14, 1995. In this disclosure, the stiffening ring contacts the subassembly only along the first and second axial ends of the stiffening ring, because the patentees, Campbell et al, claim that an even larger gap should exist between the outside diameter of the stiffening ring and the outside diameter of the subassembly.
U.S. Pat. No. 5,178,633-Peters dated Jan. 12, 1993 discloses another concept where a continuous metal band is heat shrinked onto the outside diameter of the subassembly. As disclosed by Dr. Joseph E. Shigley in his text, Mechanical Engineering Design 3rd ed., McGraw-Hill Book Co., New York, 1977, pp. 63-69, shrink fits cause significant radial and circumferential stresses in the inner member (orifice ring after shrink fit) and significant stresses can be induced in constrained bodies undergoing heating (orifice ring during shrink fit process). The disadvantage to this method of constraint is that it too requires that the orifice ring be thicker than it would if the constraint were to be applied without the press fit.
U.S. Pat. No. 4,863,460-Magladry dated Sep. 5, 1989 discloses a continuous metal stiffening ring covered by fabric, which can be electromagnetically deformed inwardly, clamping the suture ring to the subassembly. U.S. Pat. No. 4,743,253-Magladry dated May 10, 1988 is similar to the U.S. Pat. No. 4,863,460 disclosure, but utilizing a split ring. Both of these concepts present problems with manufacturing, particularly potential damage to the subassembly, acceptable stiffness characteristics, and biocompatibility.
In summary, there are several disadvantages to the current prior art design configurations of suture rings in heart valve prostheses. Some designs are inadequate because they require metal orifice rings instead of the preferred material, pyrolytic carbon. Other designs fail to maximize the internal diameter of the orifice ring, even while utilizing pyrolytic carbon. Further, some designs subject the pyrolytic carbon orifice rings to undesirable stresses and potential damage during manufacture.